It's completely unnecessary. I currently have 636 books on my Kindle, many of them large omnibus editions. I have 2250MB of storage space free, from the total 3.25GB. Honestly, if you're using the Kindle purely for reading, an SD card slot is not a requirement.

...for you. What if for example someone wants to store a library of audiobooks? Or they want to have a their own copy of Project Guttenberg available for them all the time?